FIG. 1 of the accompanying drawings shows the configuration of a conventional controller for controlling motors.
Referring to FIG. 1, the controller generally comprises a motor 6 having windings WU, WV and WW, an arithmetic unit 7 for calculating control items and generating given signals, and a power amplifier.
The power amplifier uses 2-phase AC commercial power sources R and S, and is connected to these power sources via a reactor LLP and a diode bridge DB1. The reactor LLP reduces higher harmonics of the AC power sources while the diode bridge DB1 performs full-wave rectification. A DC current and a DC voltage are supplied to a capacitor CC3 via the reactor LLP and the diode bridge DB1. When the power amplifier has a large capacity, 3-phase AC power sources will be frequently utilized.
Transistor bridges TR21 to TR26 constituting an invertor are connected to the arithmetic unit 7 via bases thereof, and are controlled by this unit 7. The invertor supplies drive currents IU, IV and IW to the windings WU, WV and WW of the motor 6 so as to drive the motor 6. These drive currents IU, IV and IW are 3-phase AC currents which differ from each other in phase by 120.degree..
Diodes are disposed between collectors and emitters of the transistor bridges TR21 to TR26 so as to supply the currents in a direction opposite to these transistors.
Electric current detectors CT2, CT3 and CT4 are respectively connected to one end each of the windings WU, WV and WW so as to detect drive currents flowing therethrough. These detectors CT2-CT4 detect the drive currents to the windings, and provide signals indicative of detected results to the arithmetic unit 7, which controls the transistor bridges TR21 to TR26 in response to these signals, thereby performing torque control of the motor.
A discharge resistor RD and a discharge transistor TR27 are juxtaposed, in this order, between an upper power source VH and a lower power source VL of the power amplifier and the capacitor CC3 is also connected between the two power sources. When the motor has large regenerative energy, a voltage VH-VL across the capacitor CC3 becomes too large. In such a case, the discharge transistor TR27 is actuated under the control of the arithmetic unit 7, and an electric discharge is performed via the discharge resistor RD and the discharging transistor TR27. Thus, the voltage VH-VL across the capacitor is maintained at an appropriate value. A current detector CT5 is connected to a collector of the discharge transistor TR27, detects a collector current, and provides a signal indicative of the detected collector current.
The arithmetic unit 7 is substantially electrically insulated from the power amplifier, receives command signals from a host control unit without via an isolator, and generates a control power source 8 for the arithmetic operation. On the basis of data concerning the drive currents flowing through the windings WU, WV and WW, the detected voltage VH-VL across the capacitor CC3 detected by the current detector CT1 and so on, the arithmetic unit 7 provides the predetermined drive signals to the bases of the transistors TR21-TR27 of the invertor, and supplies the predetermined discharging signal to the base of the discharge transistor TR27.
Outputs PTR21-PTR27 of the control power source 8 are power sources for operating transistors in the power amplifier, and are electrically insulated from one another.
The motor 6 may be an induction motor, a synchronous motor, a reluctance motor, or the like. The controller as shown in FIG. 1 functions to enable the motor to operate without any problem.
The motor controller configured as shown in FIG. 1 can function without any problem, but is somewhat disadvantageous in that it is expensive and bulky.
Referring to FIG. 1, a number of transistors are used to drive the windings of the motor, which means that it is rather difficult to down-size the controller.
Since the arithmetic unit 7 is electrically isolated from the power amplifier, insulated power sources are required and are connected to respective transistors so as to operate them. Electrical insulation between the arithmetic unit 7 and the power amplifier necessitates disposing the current detectors and voltage detectors at specified portions of the power amplifier so as to operate the motor effectively. This makes it difficult to down-size the controller. Further, detector isolators are required as the detectors are rather expensive, which tends to make the controller disadvantageous due to its increased cost.
The transistor driving signals have to be transmitted to the transistors via isolators such as photo-couplers, which are required for the respective transistors.
When excessive power is regenerated in the motor and the power is supplied to the power source VH during the regeneration of the motor, a discharge circuit is necessary so as to enable the power source VH to have an appropriate potential. Since a large current flows through the discharge resistor of the discharge circuit, the controller as well as the discharge resistor is prone to a problem that it is heated.
The foregoing controller rectifies commercially supplied power so as to obtain drive power for the transistors. Since the commercially supplied power has a relatively large voltage fluctuation, the transistors should have a voltage rating which is compatible with such a voltage fluctuation of the commercial power. Thus, the conventional controller suffers from the foregoing problem.
It is conceivable to down-size the foregoing controller by reducing the number of transistors as power elements. For example, the motor windings are connected to a high voltage power source at ends thereof, and to power elements at the other ends thereof. Then, power is supplied from the high voltage power source to the power elements. Such a controller is rather ineffective compared with the controller shown in FIG. 1. Therefore, there have been strong demands for less expensive but more reliable controllers.